Get the first cell in each column

I've been trying to get PHPExcel to read each cell in the first row of a spreadsheet. A problem that I have is I cannot iterate through columns because the columns are labeled with letters and I do not know how to iterate through letters. If there was a
way to map numbers to column titles (so that 26 maps to Z, 27 to AA, etc.) it would be helpful.

Another problem is that I can only get the last column in terms of a letter, so I cannot make a comparison to determine if the current column of the iteration is less than the last column of the iteration.

How do I read row 0? The $row variable was defined during iteration, so if I didn't iterate over the row, $row would be undefined. Is it something like $objWorksheet->getRowIterator()[0] or $objWorksheet->row[0]?